arXiv subjects

N. Evans

Publications and source records attributed to N. Evans.

25 records · Page 2Linked to original sources

arXiv1997

Radiation damage to neutron and proton irradiated GaAs particle detectors

The radiation damage in 200 um thick Schottky diodes made on semi-insulating (SI) undoped GaAs Liquid Encapsulated Czochralski (LEC) bulk material with resistivities between 0.4 and 8.9*10E7 Ohm*cm were studied using alpha-spectroscopy, signal response to minimum ionising particles (MIP), I-V and CV-measurements. The results have been analysed to investigate the influence of the substrate resistivity on the detector performance after neutron and proton irradiation. The leakage current density, signal response to alpha-particles and MIPs show a strong dependence on the resistivity before and after irradiation. An observed decrease of the electron mean free drift length before and after irradiation with increasing substrate resistivity can be explained by a model involving the different ionisation ratios of defects, which are introduced by the irradiation. Comparison of the radiation damage due to neutrons and protons gives a hardness factor of 7+-0.9 for 24 GeV/c protons. The best detectors show a response to MIPs of 5250 e- at 200 V reverse bias after a irradiation level of 2*10E14 p/cm^2.

hep-ex↗

arXiv1997

Analysis of Trapping and Detrapping in Semi-Insulating GaAs Detectors

To investigate the trapping and detrapping in SI-GaAs particle detectors we analyzed the signals caused by 5.48 MeV alpha particles with a charge sensitive preamplifier. From the bias and temperature dependence of these signals we determine the activation energies of two electron traps. Additional simulation and measurements of the lifetime as a function of resistivity have shown that the EL2+ is the dominant electron trap in semi-insulating GaAs.

hep-ex↗

arXiv1997

Radiation studies for GaAs in the ATLAS Inner Detector

We estimate the hardness factors and the equivalent 1 MeV neutron fluences for hadrons fluences expected at the GaAs positions wheels in the ATLAS Inner Detector. On this basis the degradation of the GaAs particle detectors made from different substrates as a function of years LHC operation is predicted.

hep-ex↗

arXiv1996

QCD at Large $θ$ Angle and Axion Cosmology

We use the chiral Lagrangian to investigate the global properties of the $N$-flavor QCD vacuum as a function of the $θ$ parameter. In the case of exact quark degeneracy we find evidence for first order phase transitions at $θ= π\cdot ({\rm odd~ integer})$. The first order transitions are smoothed by quark mass splittings, although interesting effects remain at realistic quark masses. We emphasize the role of the $η'$ condensate in our analysis. Finally, we discuss the implications of our results on the internal hadronic structure of axion domain walls and axion cosmology.

hep-ph↗

arXiv1996

Phase Transitions in Softly Broken N=2 SQCD at Non-zero Theta Angle

We investigate the behavior of softly broken $N=2$ SQCD at non-zero bare theta angle $θ_0$, using superfield spurions to implement the SUSY breaking. We find a first-order phase transition as $θ_0$ is varied from zero to $2 π$, in agreement with a prediction of `t Hooft. The low-energy theta angle $θ_{eff}$, which determines the effective charges of dyonic excitations, has a complicated dependence on $θ_0$ and breaking parameters.

hep-th↗

arXiv1996

Chiral Perturbation Theory, Large-N_c and the eta' Mass

Using chiral perturbation theory and the large-$N_c$ expansion, we obtain expressions for the $η'$ mass and $η- η'$ mixing in terms of low-energy chiral Lagrangian parameters. This is accomplished through an intermediate step of `matching' the topological susceptibility in the large-$N_c$ and chiral Lagrangian descriptions. By inserting the values of well-measured parameters we obtain predictions involving the the second order parameters $L_6,L_7$ and $L_8$. The prediction for $L_6$ is quite restrictive even after allowing for $1/ N_c$ corrections.

hep-ph↗

arXiv1996

Topological Charge and $U(1)_A$ Symmetry in the High Temperature Phase of QCD

We discuss the global symmetries of the high temperature phase of QCD with $N_f$ massless quarks. We show that the $U(N_f) \times U(N_f)$ symmetries are only violated by operators of dimension $\geq 3 N_f$. For $N_f > 2$ this implies that the thermal two-point correlation functions of the $η'$ and $π^a$'s are identical. We discuss the implications of this for the chiral phase transition at finite temperature.

hep-ph↗